In order to maintain swimming pool water clear and suitable for pool users chemicals must be added to the water. The most usual chemical used to control undesired organic growth in the pool is chlorine. The chlorine is derived from granular sodium hypochlorite. This should be introduced into the pool in dosed quantities and at regular intervals. Usually this granular materiel is introduced by hand though there have been proposals to do this automatically.
One of the problems is to prevent moisture contacting the granules other than at the time of introduction into the pool water and to dose the water at regular intervals without the use of complicated mechanisms and electrical circuits.